1. Field of the Invention
The present invention relates to engine fuel control systems, and more specifically to the use of a smoke sensor as the basis for the control of the fuel system.
2. Related Art
Engine fuel control systems (FCS) are used not only to control the amount of fuel delivered to the engine for combustion, but also to control the amount of air being delivered to the engine for combustion, the composition of this air if the engine utilizes an exhaust gas recirculation system, the amount of turbo boost pressure if the engine utilizes a turbocharger having a wastegate or variable geometry, and various other emissions and/or performance related functions such as swirl-port deactivation and fuel injector operation.
Fuel control systems for gasoline engines are typically closed-loop in nature, using feedback from a switching-type oxygen sensor disposed within the engine exhaust system as one basis of its control. These fuel control systems have significant shortcomings, however, when incorporated into lean-burn type engines, such as diesel engines, where smoke is more readily produced and is therefor of greater concern. As used herein, the term `smoke` shall be defined to include particulate matter, black smoke and white smoke. As is known in the art, particulate matter consists of all non-gaseous substances, excluding unbound water, which are normally present in the exhaust gases. Particulate matter from a diesel engine, for example, typically consists of combustion-generated solid carbon particles, commonly referred to as soot, with occluded organic and inorganic compounds such as unburned hydrocarbons, oxygenated hydrocarbons, sulfur dioxide, nitrogen dioxide and sulfuric acid. Visible air-borne particulate matter is commonly referred to as black smoke. White smoke is comprised of highly atomized unburned fuel.
When controlling the fueling of a lean-burn type engine, the fuel control systems used with traditional gasoline engines exhibit significant shortcomings that stem from the fact that oxygen is a poor indicator of the smoke level in the exhaust and that the exhaust typically contains large amounts of oxygen regardless of the performance or emission level of the engine. Transitions from one emission/performance state to another can be accompanied by relatively small changes in the oxygen level of the exhaust. Consequently, a diesel engine fuel control system whose control is based upon the level of oxygen of the exhaust cannot identify the current emission /performance state of the engine with complete accuracy.
Because fuel control systems utilizing O2 sensors have difficulty detecting the onset of an undesired emission/performance state, the transient response of these systems usually includes a significant time lag between the point at which the undesired emission state is identified or predicted and the point at which the corrective action is implemented (e.g. the EGR valve is closed and the air manifold is purged of any intake air containing recirculated exhaust gases). As such, the transient response of a fuel control system whose control is based upon the level of oxygen in the exhaust is extremely poor.
Another drawback of fuel control systems that utilize O2 sensors is that oxygen is not a "direct" indicator of either smoke or NOx. Consequently, in developing the calibration of the fuel control system, a set of assumptions must be made to relate the oxygen level to the levels of smoke and NOx. As many of the variables which contribute to the production of smoke and NOx are not directly monitored by the engine, a relationship must be derived associating the effects of these variables on the level of oxygen in the exhaust. Where no relationship between a variable and the amount of oxygen is found to exist, the "worst-case" effects of the variable are factored into the FCS calibration.
Many of these variables are the result of part-to-part variations stemming from ordinary design issues, manufacturing and machining tolerances or wear. Some of these variables can effect the performance and operation of the engine as a whole, such as differences in the output of a fuel pump, fuel injector, or turbocharger. Other variables are apparent only when differences in cylinder-to-cylinder emissions and performance are analyzed. These variables include, for example, differences in the output of fuel injectors, pressure drops across the length of the air manifold, volumetric differences between the cylinders, and timing differences between the opening of the valves.
Although these assumptions are necessary to ensure that desired emissions levels are maintained, they have several drawbacks. Due to the multitude of variables involved, the effort to calibrate the fuel control system is immense, consisting of the identification of each variable, an analysis of the impact of the variable on the performance and emissions output of the engine and an analysis of the best way to incorporate the effects of the variable into the calibration of the fuel control system.
The resulting calibration is a compromise between the competing interests of performance and emissions output, with the emphasis being placed on maintaining emission levels within desired limits. As many of the variables cannot be directly monitored or have no direct relationship with the level of oxygen in the exhaust gases, performance is sacrificed, not only because the characteristics of the engine will likely deviate from the "worst-case" scenario used to develop the calibration, but also because a margin of safety is incorporated into the calibration which sacrifices performance in order to ensure that the emissions output is maintained below desired limits.
Although a fuel control system using closed-loop feedback control based on the level of oxygen in the engine exhaust is technologically possible (e.g. wide-range type oxygen sensors) and although these systems are generally capable of providing better overall control than the presently used open-loop controls, such systems have not received commercial acceptance due to the additional cost associated with such systems. As such, the fuel control systems for diesel engines are typically operated on an open-loop basis. These open-loop systems have limited means for making changes in response to the actual emissions/performance of the engine in both the manner and amount of fuel and air that is being delivered to the engine for combustion. The ability to detect or monitor certain parameters typically requires additional sensors. For example, a barometer sensor is frequently incorporated into the prior art open-loop fuel control systems to allow for compensation for variations in the ambient air pressure caused by changes in the weather and/or by the altitude at which the engine is being operated. These pressure changes have a proportional effect on the air-to-fuel ratio being delivered to the engine and as such, compensation is necessary to ensure that the desired mass flow of air is being delivered to the engine for combustion. Additional sensors could be included to monitor a multitude of various characteristics that effect the emissions and performance of an engine, but this would increase both the cost and complexity of the fuel control system without providing the ability to optimize the engine performance while maintaining emissions outputs within desired levels. These open-loop systems have a very limited capability to perform on-board diagnostics to identify instances where the emissions output of the engine exceeds a predetermined level and/or malfunctioning equipment impacting the emissions/performance of the engine.
Therefore, there remains a need in the art for a fuel control system for a lean-burn engine that operates with closed-loop feedback to regulate the amount of air and the amount and timing of fuel delivered to the engine for combustion to improve performance, reduce emissions and provide improved on-board diagnostics for equipment critical for the control and monitoring of the engine exhaust emissions.
The present invention provides a fuel control system using closed-loop feedback control based on the level of smoke in the engine exhaust. Closed-loop feedback control allows various aspects of the delivery of air and fuel to the engine for combustion to be tailored to the characteristics of an individual engine allowing performance to be optimized while maintaining emissions within desired limits.